Method for the transportation of crystals and melt

ABSTRACT

Crystals and melt are transported in a crystallization column in countercurrent and in temperature gradient by a rotatable spiral having a plurality of turns. The spiral is disposed within an annular gap defined by the inner wall of the column and by a rod which is disposed within the column so that its axis is coincident with the column axis. The spiral extends substantially from the rod to substantially the inner wall of the column. The turns of the spiral run downwardly upon rotation of the spiral. The spiral has at its lower end one or more turns of opposite direction. The rod is rotatable about its longitudinal axis independent of the rotation of the spiral. This rotation is preferably in counterdirection to the direction of rotation of the spiral.

United States Patent [72] Inventors m n hil kn ht 2,780,663 2 1957Gunness 23/273 F Wilcken Str. 5; 2,839,411 6/1958 Vela 62/58 Klaus Mans,Zuincke Str. 44, both of 2,854,494 9/1958 Thomas 62/58 Heidelberg,Germany 2,920,347 1/1960 Joukainen 18/12 SF [21] App]. No. 875,9563,101,598 8/1963 Ross 62/354 [45] Patented Dec.28, 1971 1 161852 [H964 G[32] Priorities Nov- 15, 1968 ermany 159/6 W [33] Germany PrimaryExaminer-Norman Yudkofi [31] P 18 09 104.8; Assistant Examiner-S.Silverberg Nov. 15, 1968, Germany, No. P 18 09 Attorneys- Ernest F.Marmorek, Jordan B. Bierman and 105.9 Marmorek and Bierman [54] METHODFOR THE TRANSPORTATION OF ABSTRACT: Crystals and melt are transported ina crystal- CRYSTALS AND MELT lization column in countercurrent and intemperature gradient 7 Claims, 1 Drawing Fig. by a rotatable spiralhaving a plurality of turns. The spiral is disposed within an annulargap defined by the inner wall of the :fil column and y a rod which isdisposed within the column so that its axis is coincident with thecolumn axis. The spiral ex- [50] Field of Search ..23/273, 273

F 270 301' 62 tends substantially from the rod to substantially theinner wall 18/12 SF of the column. The turns of the spiral rundownwardly upon rotation of the spiral. The spiral has at its lower endone or [56] References Cited piorettuirns pf OPPOCSlIC digecttiorti kghertoil is rott attlable abpult ongi u ina axis in epen en 0 e re a ion 0e splra 15 UNITED STATES PATENTS rotation is preferably incounterdirection to the direction of 2,540,706 2/1951 Fishill 259/107rotation fth SpiraL 3,433,598 3/1969 Faaborg-Andersen 23/270 7D/sCl/dfif 7 3 Pd/A/f 4 40 7 WW V :Zf" \.4 2

8 7 EV 7-Hi47'5/i DISCHAE 0 pa/A/T I 2 7 7 6 \5 METHOD son THETRANSPORTATION or CRYSTALS AND MELT The present invention relates to amethod for transporting crystals and melt in a crystallization column incountercurrent and in temperature gradient. The invention furtherrelates to a crystallization column suitable for carrying out thismethod.

The transportation of crystals and melt in a crystallization column incountercurrent and in temperature gradient by means of a spiral (helicalspring) which rotates in and fills entirely an annulai gap defined bythe inner wall of the column and a fixed rod which is disposed withinthe column so that its axis is coincident with the column axis, thespiral rotating in such a way that the turns of the spiral run downward(i.e., toward the better part of the column), is known (H. Schildknechtand H. Vetter, Angewandte Chemie: 73: 612-615 (1961). According to thismethod, large quantities of a substance can be put through the columnper unit time, and the continuous preparation of very pure substances ispossible, even with mixed-crystal systems.

A new method has now been found which is much more efficient than theknown method. in this new method, crystals and melt are transported in acrystallization column in countercurrent and in temperature gradient bymeans of a spiral which rotates in the annular gap defined by the innerwall of the column and a rod which is disposed within the column so thatits axis is coincident with the column axis. The rotation of the spiralis such that the turns of the spiral run downward (i.e., toward thehotter part of the column). The spiral fills substantially the entireannular gap. This new method is characterized in that:

a. at its lower end (i.e., that end extending into the hot part of theannular gap) the spiral has one or more (preferably one to two) turns ofopposite direction, which, therefore, upon rotation of the spiral, runupwardly, and

b. the rod which is disposed within the column so that its axis iscoincident with the column axis, is rotatable about its longitudinalaxis.

The present invention further relates to a crystallization columnsuitable for carrying out the method of the present invention. Thisapparatus requires much less expenditure for control and servicing ascompared with the known apparatus.

The new crystallization column, the spiral and the rod, which isdisposed within the column so that its axis is coincident with thecolumn axis, may, depending on temperature conditions and the type ofcrystal-melt systems to be separated, consist of ordinary materials.Refractory (heat-resistant) materials, such as special steel, e.g., VA(Material No. 4541) or VA (Material No. 4571) are, however, preferred.

The surfaces of the annular gap (that is, of the inner wall of thecolumn and of the hollow or full rod, as defined) may be smooth orroughened, and may be provided with longitudinal or spiral grooves.Generally, they are smooth.

The rod which is disposed within the column so that its axis iscoincident with the column axis preferably rotates about itslongitudinal axis during the entire transportation process.

Advantageously, it rotates in opposite direction to the direction ofrotation of the spiral. The rod may also be rotated back and forthalternately. By this rotation of the rod, one obtains the possibility ofrunning harder, that is, of using a denser crystal paste than would bepossible with the use of an otherwise identical apparatus which,however, uses a fixed rod. This in turn permits an approximately threeto six times greater throughput per unit time. (Denser" crystal paste,as used herein, means a higher proportion of crystals in thecrystal-melt system).

The preferred dimensional ratios of the crystallization column are asfollows:

The ratio of the inside diameter of the column to its length isapproximately 1:4 to 1:50, particularly 1:15 to 1:30.

The ratio of the outside diameter of the rod to the inside diameter ofthe column is in the order of about 1:2. lt may, however, vary greatlydepending on the size of the apparatus,

the substance systems to be separated, and temperature conditions.

Although the pitch of a turn is advantageously equal to the differencebetween the inside radius of the column and the outside radius of therod, it may also be double or one-half this value. The pitch of thecounterturn is generally the same as that of the turns.

The speed of rotation of the spiral is adapted to the particularconditions encountered. It is especially dependent on the viscosity ofthe crystal-melt system and on the dimensions of the spiral (diameter,pitch). The speed of rotation of the spiral is usually about 10450revolutions per minute. If the rod which is disposed within the columnso that its axis is coincident with the column axis also rotates, itsspeed of rotation is usually two to five times greater than the speed ofrotation of the spiral.

The point of introduction for the raw material to be separated isadvantageously located at approximately the middle of the column. Thedischarge point of the lower melting substance (or substance system) isadvantageously located at the upper (i.e., coldest) end of the column.

Due to the novel counterturn(s) at the lower end of the spiral, thecrystal paste is considerably densified at the level of the turnreversal in the annular gap. The residual melt adhering to the crystalsis squeezed off the crystals. Thus there forms at the turn reversal acrystal plug which consists of crystals of the higher melting product.To tap this product, heat is supplied to partially melt the plug. A partof this melt is tapped. The rest of the melt passes back into thecountercurrent. It is therefore obvious that the tapping point at thecolumn is advisably disposed at the level of the turn reversal.

The hottest point of the column thus lies at the level of the turnreversal. From that point, the temperature deci'eases steadily,particularly toward the top part of the column. One produces thistemperature gradient in the usual manner by adequate heating, cooling,and heat insulation.

It should be noted that it is possible to connect several such columnsone behind the other in cascade operation.

The present invention will now be explained with reference to thefollowing examples, as well as to the drawing, in which:

The FIGURE is a schematic view partly in section of a crystallizationcolumn which embodies the method of the present invention.

EXAMPLE 1 With reference to the drawing, 1,500 g. of molten crudecaprolactam per hour are pressed through filling tube 1 of thecrystallization column of the FIGURE into annular gap 2. The column hasa useful length (annular gap) of about 90 cm. and an inside diameter of8.2 cm. A fixed hollow rod 3 having an outside diameter of 3.6 cm. isdisposed in the column so that its axis is coincident with the columnaxis. A spiral 4 having 28 turns (of which only five are shown), onecounterturn 5, and a pitch of 3.0 cm. is disposed within annular gap 2.This spiral 4 is rotated at a speed of 30 revolutions per minute, insuch a way that the 28 turns run downward, and the counterturn 5 runsupward, the crystals of caprolactam thereby being conveyed into theseturn directions. A crystal plug forms atthe turn reversal 6. The formedcrystal plug is partially melted by means of a heating device 7. A partof this melt is tapped through the discharge point for the highermelting fraction 8. The rest of the melt again passes into thecountercurrent. The column is insulated with an insulating layer 10.

The amount of material removed at the discharge point for the highermelting fraction 8 and at the discharge point for the lower meltingfractions 9 is equal to the amount of crude product introduced into thecolumn through filling tube 1.

The column operates automatically, without requiring checking orservicing. The yield of completely colorless pure caprolactam obtainedis percent based upon the crude product.

EXAMPLE 2 The spiral 4 rotates at revolutions per minute. The rod 3rotates in opposite direction to the spiral at 50 revolutions perminute, and the throughput of crude caprolactam is 5,000 g./hr. Theother conditions are the same as in example 1. The same is true of theyield of pure caprolactam.

What is claimed is:

l. In a method for transporting crystals and melt in countercurrent flowto one another and through a temperature gradient between a cooler partand a hotter part in a crystallization column having a spiral which hasa plurality of first turns running in a predetermined direction and oneor more second turns located in the hotter part of the said column andrunning in a direction opposite to the direction of said first turns, arod within the column having its axis coincident with the axis of saidcolumn, the steps comprising introducing raw material containing saidcrystals and melt into said column, rotating said spiral in a directionsuch that said first turns move the crystals of the raw material towardthe hotter portion of the column and in which the said second turnsadvance the crystals and the residual melt adhering to the crystalstoward the cooler portion to densify material in the region where saidfirst and second turns are adjacent each other, and rotating said rodindependently of the rotation of said spiral.

2. A method as described in claim I wherein the rotation of the rod isindependent of the rotation of the spiral.

3. A method as described in claim I wherein the rotation of the rod isin counterdirection to the direction of rotation of the spiral.

4. A method as described in claim I wherein the direction of rotation ofthe rod is periodically alternated.

5. In a crystallization column wherein crystals and melt are transposedin countercurrent flow to one another and through a temperature gradientbetween a cooler part and a hotter part by a rotatable spiral having aplurality of turns, the improvement comprising one or more turns of saidspiral at one end thereof having a pitch opposite in direction to thepitch of the remaining turns, and a rotatable rod disposed within thecolumn so that its axis is coincident with the column axis, said rodbeing rotatable independently of said spiral, said spiral surroundingsaid rod and extending substantially from the rod to substantially theinner wall ofthe column.

6. A crystallization column as described in claim 5 wherein the rodrotates about its longitudinal axis in a direction counter to thedirection of rotation of the spiral.

7. A crystallization column as described in claim 5 wherein thedirection of rotation of the rod about its longitudinal axis isperiodically alternated.

1. In a method for transporting crystals and melt in countercurrent flowto one another and through a temperature gradient between a cooler partand a hotter part in a crystallization column having a spiral which hasa plurality of first turns running in a predetermined direction and oneor more second turns located in the hotter part of the said column andrunning in a direction opposite to the direction of said first turns, arod within the column having its axis coincident with the axis of saidcolumn, the steps comprising introducing raw material containing saidcrystals and melt into said column, rotating said spiral in a directionsuch that said first turns move the crystals of the raw material towardthe hotter portion of the column and in which the said second turnsadvance the crystals and the residual melt adhering to the crystalstoward the cooler portion to densify material in the region where saidfirst and second turns are adjacent each other, and rotating said rodindependently of the rotation of said spiral.
 2. A method as describedin claim l wherein the rotation of the rod is independent of therotation of the spiral.
 3. A method as described in claim l wherein therotation of the rod is in counterdirection to the direction of rotationof the spiral.
 4. A method as described in claim l wherein the directionof rotation of the rod is periodically alternated.
 5. In acrystallization column wherein crystals and melt are transposed incountercurrent flow to one another and through a temperature gradientbetween a cooler part and a hotter part by a rotatable spiral having aplurality of turns, the improvement comprising one or more turns of saidspiral at one end thereof having a pitch opposite in direction to thepitch of the remaining turns, and a rotatable rod disposed within thecolumn so that its axis is coincident with the column axis, said rodbeing rotatable independently of said spiral, said spiral surroundingsaid rod and extending substantially from the rod to substantially theinner wall of the column.
 6. A crystallization column as described inclaim 5 wherein the rod rotates about its longitudinal axis in adirection counter to the direction of rotation of the spiral.
 7. Acrystallization column as described in claim 5 wherein the direction ofrotation of the rod about its longitudinal axis is periodicallyalternated.